Job costing starts with structure. Break each project into phases that match how you estimate and build: site work, foundation, framing, roofing, mechanicals, finish work. Within each phase, separate labor, materials, and subcontractors. This structure lets you compare actual costs to your estimate at a level detailed enough to spot problems while you can still do something about them.

Every expense needs to hit a specific job and phase. When you buy lumber, it goes to framing materials on that project. When a plumber invoices you, it goes to mechanicals subcontractor on that job. When your crew logs hours, those hours get split by phase if they worked on multiple things that day. No exceptions, no miscellaneous bucket, no dumping costs into general overhead because coding seems tedious.

Track labor by phase, not just by day. A crew that spends the morning on framing and the afternoon on trim needs their time split accordingly. This feels like extra work until you realize you’ve been underpricing interior finish for years because all labor was lumped together.

Subcontractor invoices often represent half or more of a project’s cost. Get them coded correctly before you pay. If sub invoices just hit a general expense account, your job costing numbers are fiction.

Compare budget to actual weekly during active construction. Monthly reviews mean you discover the framing overrun after the house is dried in. Weekly reviews catch it while adjustments are still possible. Build a simple report showing budgeted cost by phase versus actual plus committed costs.

Committed costs matter. You signed a contract with an electrician for $28,000 but have only received invoices for $12,000 so far. Your spent-to-date looks fine, but you’re actually over budget once the remaining invoices come through. A bookkeeper for small business construction work will track commitments alongside payments.

Your chart of accounts needs to support this structure. Generic QuickBooks setup doesn’t work for contractors. You need job costing enabled and accounts organized to capture costs the way work actually happens in the field. Most contractors who set this up themselves end up with a system that can’t produce useful job-level reports.

The payoff is knowing real margins by project and by phase. You stop bidding certain work too low because you can see it runs over consistently. Your estimates improve because they’re built from actual cost history. Job costing for contractors done right turns your books from a tax compliance exercise into a tool for making better decisions on every bid.